Donald Trump suggested that if Hillary Clinton gets to pick US federal judges as president, there's nothing that can be done to protect the right to bear arms.
But then he added without elaboration that maybe supporters of the second amendment could figure out a way.
He said on Tuesday at a rally in North Carolina: "By the way if she gets to pick her judges, nothing you can do, folks. Although the second amendment people, maybe there is, I don't know."
It wasn't immediately clear what Trump meant by his remarks, at a rally in Wilmington, North Carolina - his spokeswoman didn't immediately reply to a request for a clarification. But the second amendment provides a constitutional right to citizens to own guns.
And Democrats have attacked the US presidential nominee after the comments, with Democratic Party members accusing the billionaire Republican of openly encouraging violence against his election opponent.
Clinton campaign manager Robby Mook said: "This is simple -- what Trump is saying is dangerous. A person seeking to be the president of the United States should not suggest violence in any way."
Tim Kaine, the Democratic vice presidential nominee, said: "I think it was just revealing and I don't find the attempt to roll it back persuasive at all."
Priorities USA, a super PAC - expenditure-only committee - supporting Clinton, said Trump had "suggested that someone shoot Hillary Clinton".
Meanwhile, The Brady Campaign to Prevent Gun Violence, which has endorsed Clinton, said Trump was encouraging gun violence "based on conspiracy theory about Hillary Clinton".
But the National Rifle Association, the gun lobby that has endorsed Trump, came to his defence, writing on Twitter that "there's nothing we can do" if Clinton is elected, urging voters to defeat her in November.
At another rally later in Fayetteville, North Carolina, Trump was careful with his words. He repeated his argument that Clinton posed a threat to gun rights, but avoided any talk about advocates taking matters into their own hands.
The US Secret Service, responsible for both Clinton's and Trump's protection, said it was aware of what Trump had said, but declined to say whether it planned to investigate.
Contrary to Trump's remarks, Clinton has made her support for gun rights a key piece of her stump speech in a bid to pre-empt attacks from Trump and groups like the NRA.